i have WPMU installed and working since 2 years. Had zingiri forum 1.0 installed then. Recently had to uninstall 1.0 version due to cache flooding issue. Now i have installed 1.1 whcih has automatic cache clearing. Now the problem i am facing.
When i click on Administration (in the backend) it asks me for a userid and password. I have tried all combinations viz, default, database, administrator but nothing works. Please help.

503 errors are not coding related, they're site set up related. It could also be the case that the domain name is not resolving properly on the server itself. It's best to check the web server logs, that should provide more info about the 503 error. You can always ask your hoster, they should be able to help with this.

thank you so much for your reply. But the thing is i have tried my super admin credentials, default admin credentials and the one supplied in the "integration" window, nothing works. I request someone to please look into this issue.